Dynamically Hydrogen-Bonded Microphase Separation Enabling Phase Transition in the Gel Composites With Tunable UCST

Area of Science:

• Materials Science
• Polymer Chemistry
• Soft Matter Physics

Background:

• Thermal-responsive hydrogels are widely used but tuning their phase transition temperature, especially for UCST-type hydrogels, remains challenging.
• Existing hydrogels often lack the tunability required for specific applications, necessitating novel material designs.

Purpose of the Study:

• To design and characterize a novel polyacrylamide (PAm)-tannic acid (TA) composite hydrogel exhibiting a tunable UCST-type phase transition.
• To elucidate the physical mechanisms governing the phase transition behavior and microstructure evolution.
• To explore potential applications in smart devices through tunable phase transition properties.

Main Methods:

• Synthesis of polyacrylamide-tannic acid composite hydrogels.
• Characterization of phase transition behavior and tunability of the Upper Critical Solution Temperature (UCST).
• Investigation of microstructures and dynamics using Very Small Angle Neutron Scattering (VSANS), dynamic light scattering, and pulsed-field-gradient NMR.
• Analysis of viscoelastic properties and their correlation with microstructural changes.

Main Results:

• The PAm-TA hydrogel demonstrated a tunable UCST-type phase transition over a wide range.
• Microphase separation driven by reversible hydrogen bonding between TA and PAm was identified as the key mechanism.
• Dynamical studies revealed increased TA diffusion and accelerated relaxation dynamics at higher temperatures due to weakened hydrogen bonding.
• A programmable information encryption/decryption device was successfully designed using the tunable UCST property.

Conclusions:

• The PAm-TA hydrogel offers a new platform for designing smart materials with precisely controlled phase transition temperatures.
• The findings provide insights into the interplay between hydrogen bonding, microstructure, and macroscopic properties in responsive hydrogels.
• This work opens avenues for applications in optical devices, environmental sensors, and information storage.